Let me emphasise, GeeTee, that although I appreciate one or two basic things about MTU, I don't consider myself an expert in this area. After all, why would I be posting this thread if I already knew all the basic answers?

Let me repeat what I described much earlier, though. The value of MTU at which a ping to a website of my choice returned non-fragmented packets was 1432. Any value higher than that caused fragmentation. So yes, you might well ask why I've been thinking of configuring my router to a value higher than 1432 + 28 = 1460. Well, the only reason is that everyone else has been telling me that I should set my router's MTU to be the same as Windows's MTU, that being notionally 1500 (1472 + 28). On the basis that that 1500 is the maximum MTU value and not necessarily the actual MTU in use, it's seemed logical to try router MTU values of between 1460 and 1500 - since the golden rule is to get the router's MTU to be either exactly the same as Windows's MTU, or to have it slightly larger; packets then don't get fragmented as they pass through my router. Apparently, you should, in any event, never go above 1500. But maybe this is screwy logic?

The author of the article on MTU at kitz.co.uk publishes a long list of empirical Windows MTUs, obtained from various (perhaps unreliable) sources, and none of them are anything as low as 1390.

By the way, do you mean actually 1390 + 28, as the MTU for Windows?

I've reflected on what you say about RWIN and I think you're possibly correct about Windows's MTU value not automatically adapting, but the RWIN value adapting instead. So yes, maybe I got that misconstrued.

You may also be correct about any number of routers between my PC and the website blocking Path MTU Discovery, but certainly what my ISP has eluded to could alone be the cause of the overall MTU in my setup not being optimum at present, namely that if the website cannot determine from my end the most suitable packet size to use, the website may well decide to simply allocate the Windows default size of 1500 and, with my router's MTU currently being at present only 1460, a mismatch will occur. It may well be that the only way to allow the website to use Path MTU Discovery is if ICMP at my end is enabled (allowing pinging of my router by websites). However, enabling ICMP (at least, on a permanent basis) raises a potential security issue. Thus, maybe I'm left with a strict choice between either leaving ICMP disabled and accepting that the website will just throw 1500-size packets at my PC and a good deal of them getting fragmented, or that I enable ICMP and thereby throw away the stealthed nature of my connection. Either way, though, I'd still need to know what value for MTU I should set in my router. I'm still not convinced that 1500 is the correct value to use. And 1390 + 28 also seems unlikely; if the Windows MTU were that value, then with 1460 being the value currently in my router, why would I be having problems still?

Surely, the way I'm running my ping test at present is valid? It's involving all elements of a connection between my PC and a remote website, so it's very much a realistic scenario. If it's giving me the answer 1432 (+28) as being the optimum unfragmented end-to-end size, then what can be gained by setting the router's MTU to something that's considerably lower than that? Or is that that particular point of yours is more about more-completely filling the packets than avoiding fragmentation? (In one respect, this is a 'chicken and egg' situation, isn't it?).

Edited by deleted (Thu 19-Nov-09 00:52:55)

what was that tweak then.

if you do the following.

set the mtu on router to 1500 and disable any mtu clamping type feature.
set the mtu on the pc to 1500 or below and keep mtu discovery enabled.

you should never be in a situation where you dont have the ability to send data (assuming no other fault), maybe reduced performance if the 1500 isnt optimal but not no connectivity.

windows xp still defaults to 1500, meaning if you tweak to 1500 you actually changing nothing.

the default in xp sp3 is to have a 1500 mtu, icmp mtu discovery enabled and blackhole mtu discovery enabled. The default mtu changes if you are not using an ethernet connection from the pc, such as dialup modem or wireless.

if you set to 1500 and turn off mtu discovery, windows then ignores the setting and forces a 576 mtu value.

Edited by Chrysalis (Thu 19-Nov-09 08:49:10)

In reply to a post by meditator:

Which version of Windows are you using, zebedee? In my version of WinXP, there is no such parameter as EnablePMTUDiscovery in that location.

I'm using XP Professional SP3.

I have to tell you, zebedee, that EnablePMTUDiscovery, or viewing an MTU value in the Registry, is not available in Windows XP Home, which happens to be what I'm using (that's with SP3).

Chrysalis,

My guess is that your advice here is based on the use of a utility such as Dr TCP. Furthermore, I reckon that, along with several other contributors, you're assuming that both versions of WinXP (Pro and Home) can be dealt with in the same way. I'm afraid that isn't the case, as no MTU or MTU Discovery settings are made available in the Windows Registry in WinXP Home (which is what I have). There's no way that I can alter the Windows MTU, even if I wanted to and, as you've realised yourself, Windows would ignore any new setting for MTU anyway.

It appears that, when you run under WinXP Home, you've fewer options at your disposal when dealing with this issue. I'm beginning to conclude that probably the only sensible thing I can do is to simply assume that the Windows MTU is indeed 1500 and so configure my router's MTU to this same value. I think yarwell reckoned it worked okay with Netgear routers.

Edited by deleted (Thu 19-Nov-09 11:56:31)

FWIW As I mentioned in the first reply in this thread 1500 works for me!

Why not give it a try? You can always change it back...

Edited by b4dger (Thu 19-Nov-09 13:09:03)

Yes, as my slightly earlier reply indicated, I've now changed my router MTU to 1500. Thus far, no problems.

When I now ping a remote website with an MTU value of anything greater than 1472, it fragments. (Before, the value was 1432). I guess this agrees with Windows's MTU and the router MTU both being 1500 now, ie. 1472 + 28 = 1500.

In reply to a post by meditator:

My guess is that your advice here is based on the use of a utility such as Dr TCP. Furthermore, I reckon that, along with several other contributors, you're assuming that both versions of WinXP (Pro and Home) can be dealt with in the same way.

You're assuming an awful lot ! You asked how to switch path discovery on in XP, and you were told how to do it. We have no way of telling which version of XP you are using, and no way of knowing that you are using a version of XP which is not really designed for people tinkering with the TCP/IP settings. It's rather like buying a Ford Kaa, and complaining that you can't find the switch for the Xenon headlamps

yes as yarwell said never lower your router mtu, always control it at the end point (usually the pc).

regarding your xp home comment, then that is confusing. I have never heard of such a limitation and even just now a quick google shows nothing indicating xp home has no ability to adjust mtu values or the mtu discovery setting. By the way the mtu values are not there by default even in XP pro, they have to be created.

Edited by Chrysalis (Thu 19-Nov-09 14:30:36)